HOBSON: But first we're learning more about the threat that forced the State Department to ask U.S. citizens to leave Yemen today. The warning cited continued potential for terrorist attacks and an extremely high security threat level. This of course follows days of embassy lockdowns across the Middle East and Africa. And this morning, the U.S. Air Force transported State Department personnel out of Yemen's capital, leaving only the most essential employees on the ground to monitor the security situation there.

Joining us now for more on this is NPR's national security editor Bruce Auster. And Bruce, first tell us what more we know about the threat that caused this evacuation.

BRUCE AUSTER, BYLINE: Hi, well, the threat has been pinpointed because the United States apparently intercepted an electronic communication, and this happens all the time, but what was really interesting in this case was who the people were who were communicating. And they were two of the highest ranking people in al-Qaida. And again that's unusual.

So one of them was Ayman al-Zawahiri. He is the head of the organization. He replaced bin Laden. He operates out of Pakistan, obviously that's where the drone strikes are, but he's the head of the organization. The other person is the head of al-Qaida's arm in Yemen, and he has now apparently become sort of the deputy of the whole organization, but the two of them were caught in some way communicating about a possible plot.

And there was a good deal of specificity. There was a sense that the plot was imminent. There was some sense of location. But there was a sense that it could happen as early as this past Sunday. And so that is the reason that we've seen this really remarkable closing of embassies and the evacuation of personnel.

HOBSON: And the evacuation orders also come just hours after four men suspected of being al-Qaida members were killed by a suspected drone strike in Yemen. Is there a connection there?

AUSTER: Yeah, what we're trying to figure out is whether there's a connection with those four people. So there was this drone strike, four al-Qaida operatives apparently killed. Whether they were connected to the particular plot everybody's worried about, not clear at this stage.

What I think is clear is there has been a big step-up recently in drone strikes in Yemen. So this strike that we've been talking about, there have been four of them in just the past 10 days, and I think there is a sense that those - that he increase in the frequency of strikes in this time period suggests that it's connected to whatever plot they're worried about.

HOBSON: Well, let's talk about what's happening in Yemen. It has become one of the main centers of international terrorism. It's the headquarters of al-Qaida in the Arabian Peninsula. But it's also one of the main focuses of counterterrorism efforts.

AUSTER: That's right, and as you suggest, this is where the action is right now. You know, the Obama administration, we've heard them talk quite a lot about how al-Qaida has been decimated, but when they're talking about that, they're really talking about the old core group of al-Qaida that has been operating out of Pakistan.

But the group has metastasized, and so its affiliates have become more important, and the affiliate that's most dangerous has been this one in Yemen. And they're associated with a lot of the plots that you're familiar with: the Christmas Day bombing attempt, that was the underwear bomb; the cargo plane plot, this involved a bomb that was placed inside printer cartridges.

And the common denominator in all these cases is the sophistication of those bombs, and that is what makes this group dangerous; both their ability to get bombs onto things like airplanes but also their global ambition, their willingness to target even the U.S. homeland. And so that's where the U.S. counterterrorism efforts have been focused, and that's why you're seeing these drone strikes in places like Yemen.

HOBSON: Yeah, and we're now hearing about this idea of surgically implanted bombs. We talked about it right on this show. But is the U.S. strategy working in Yemen?

AUSTER: So here's the question, and you've asked a really good one here, and there are a lot of people who are prominent experts in this field. One of them is a guy named Gregory Johnson, who asks it like this. If the U.S. has spent the last few years targeting people in Yemen, using these drone strikes, why is that Yemen is still able to essentially get the United States to have to shut down its embassies in the region?

And the suggestion is that the strategy isn't working, that for every person you kill in a place like that, others step into their place, and so rather than actually making an impact on the organization, you're creating a new generation of enemies and really not solving the problem.